Having various advantages such as high thermal conductivity, lightweightness, etc., aluminum materials are widely used in heat exchangers and the like. For example, a heat exchanger mounted on an automobile includes components such as a tank, a tube and a fin, which are made of pure aluminum or an aluminum alloy, and these components are joined by brazing. Further, for brazing aluminum materials, a brazing sheet, on which a brazing filler material is clad at least on one side of an aluminum alloy sheet, is often used. Because the brazing sheet makes it possible to collectively braze a large number of components, the overall brazing process can be easily shortened and simplified.
As brazing methods for aluminum material, a vacuum brazing method and the NOCOLOK® flux brazing process were previously known. The vacuum brazing method is a brazing technique in which a brazing filler material composed of an Al—Si—Mg (Aluminum-Silicon-Magnesium) based alloy is used and an object to be processed is heated and brazed in a heating chamber of a brazing furnace under reduced pressure. In the vacuum brazing method, because an oxide film present on the surface of the aluminum material is broken up by Mg during brazing, a brazed joint can be formed without using flux. However, in the vacuum brazing method, it is necessary to use a vacuum brazing furnace configured to be able to reduce the pressure within the brazing chamber. Because a vacuum brazing furnace is expensive as compared to a common brazing furnace, when brazing is performed according to the vacuum brazing method, it has been problematic in that a reduction of the manufacturing cost is difficult.
On the other hand, the NOCOLOK® flux brazing process is a brazing technique in which a brazing filler material composed of an Al—Si based alloy is used, and after applying a fluoride-based flux onto the filler material, an object to be processed is heated and brazed in an inert-gas atmosphere. In the NOCOLOK® flux brazing process, a common brazing furnace can be used for the brazing. However, in the NOCOLOK® flux brazing process, if the flux application amount is insufficient, oxide films cannot be broken up sufficiently by the flux, which may cause degradation in brazeability, thereby leading to possible brazing failures in some cases. Moreover, when flux is not applied, brazing cannot be performed.
Brazing failures due to an insufficiency or lack of flux tend to occur, e.g., in the brazing of an object having a hollow portion, such as a tube of a heat exchanger, when the flux is applied after assembling the object to be processed. For that reason, flux is conventionally applied in advance to the entire surface of the aluminum material constituting the object to be processed to prevent the flux from being insufficient or lacking, which is problematic in that the brazing process becomes complicated.
Thus, as a method in which brazing can be performed according to a simple manufacturing process without using an expensive vacuum brazing furnace, a method of performing brazing in an inert gas atmosphere has been proposed that utilizes the function of Mg to break up oxide films. For example, in Patent Document 1, a method of performing brazing in an inert gas atmosphere has been proposed that uses a brazing sheet which includes a brazing filler material composed of an Al—Si based alloy and in which Mg is added at least to a brazing sheet-constituting layer other than the brazing filler material. According to this method, by forming a hollow structure such that the brazing filler material clad surface faces the interior, brazing can be performed without applying any flux onto the interior of the hollow structure.